Either the aircraft or the operation must "require" an SIC by regulation, type certificate, MEL, or according to some other FAA mandate. It is not sufficient that you want or even "require" a copilot (as in I really, really want one or the boss really, really wants one, etc.). So, you can designate him a copilot but he cannot log the time for the purpose of currency, additional ratings, etc. And without getting into a long drawn out discussion, be careful what responsibilities you may be actually or constructively abdicating in favor of an informal "copilot." One of the most important aspects of CRM is the appropriate division of both responsibility and workload. You really cannot give any responsibility - and arguably, any workload - to an unofficial copilot.